1. Field of the Invention
The present invention relates to a liquid fuel direct supply fuel cell system, a method for the operation control and an operation control device. More specifically, the present invention relates to a liquid fuel direct supply fuel cell system, which controls a concentration of the liquid fuel supplied to the fuel cell in order to operate the system at an optimal condition, a method for the operation control and an operation control device.
2. Description of the Related Art
In recent years, problems of the environment, resources, and energy are becoming increasingly important to be solved. As one solution to the problems, fuel cells are being actively developed. Of the fuel cells, a liquid fuel direct supply fuel cell system, particularly a direct methanol fuel cell using methanol as the liquid fuel, having a simple structure, making miniaturization and reduction in weight easy, using the liquid fuel composed of an organic solvent and water as main components directly for electric power generation without reforming and gasifying is expected to be available as a variety of portable power sources, such as mobile power sources and dispersed power systems, such as small power sources used as a power source for computers.
Such a direct methanol fuel cell is constituted by making an assembly of an anode, a cathode, and an electrolyte membrane composed of a polymer electrolyte having a protonic conductivity, by making a cell by placing the assembly between an anode side separator, which is arranged in the anode to supply an aqueous methanol solution as the liquid fuel, and a cathode side separator, which is arranged in the cathode to supply air as an oxidizing agent gas, and by integrating a plurality of the cells to make a generator.
According to the above described direct methanol fuel cell comprising the generator, the direct methanol fuel cell is constituted for stable operation thereof by making it possible to properly supply the aqueous methanol solution as the liquid fuel and to properly supply air as the oxidizing agent gas. In the direct methanol fuel cell properly receiving the aqueous methanol solution and air, the anode produces carbon dioxide and discharges hydrogen ions and electrons by a reaction of methanol and water and the cathode produces water by taking-in hydrogen ions and electrons from oxygen to generate an electromotive force in an external circuit. In other words, the anode discharges the aqueous methanol solution, which is not reacted, and carbon dioxide as a reaction product and the cathode discharges air, in which oxygen has been consumed, and water as the reaction product.
However, a protonic conductive polymer electrolyte membrane has properties providing easy migration of protons and easy permeation of methanol and, thus, methanol supplied to the anode partially reaches the cathode through the electrolyte membrane. As a result, it causes a potential drop in the cathode and a decrease in energy efficiency of the whole cell. In other words, increasing the concentration of methanol causes an increase in the amount of permeated methanol resulting in a remarkable drop of the potential of the cathode, a drop of the output voltage, and finally a decrease in the energy efficiency of the whole cell. On the other hand, decreasing the concentration of methanol enables a reduction in the amount of permeated methanol but, however, causes insufficient supply of methanol, which is necessary for the reaction, to the anode resulting in no generation of an electric current which finally causes a decrease in the energy efficiency of the whole cell. Therefore, for the purpose of operating the direct methanol fuel cell system under an optimal condition, an appropriate management is absolutely necessary for the methanol concentration and the supply amount of methanol to the anode.
The methanol concentration of an aqueous methanol solution in the conventional direct methanol fuel cell system is managed by a method using an electrochemical limiting current, the method using infrared absorption, a method using a change of gravity, and a method using a change of a refractive index. The method using an electrochemical limiting current is the method of preparing a cell, in which the anode is arranged oppositely to the cathode through the protonic conductive polymer electrolyte membrane, for measuring the limiting current, soaking this cell in the methanol solution of a sample for detection, and applying a constant voltage across the anode and the cathode to detect the concentration from a value of the current running in the cell used for measuring the limiting current. Next, the method using infrared absorption is for detecting the concentration on the basis that an increased methanol concentration of the aqueous methanol solution causes an increase in infrared absorption of a specific frequency. Next, the method using gravity is for detecting the concentration on the basis that the increased methanol concentration of the aqueous methanol solution causes a drop of gravity. Next, the method using refractive index is for detecting the concentration on the basis that the increased methanol concentration of the aqueous methanol solution causes an increase in the refractive index. According to a patent document 1 (Japanese re-publication of a PCT Application, JP-T-2002-520778,) it is disclosed that a sensor cell for measuring its activity is mounted on an electrochemical fuel cell including the direct methanol fuel cell.